1. Field of the Invention
The present invention relates to micromechanical devices for use in making inertial measurements. More particularly, the invention pertains to devices whose structures offer improved shielding, reduced stray capacitance and enhanced manufacturability over prior art devices.
2. Description of the Prior Art
Precision micro-mechanical devices have wide application in the fields of inertial navigation and guidance with respect to both long-range, re-usable vehicles, such as aircraft, and relatively short-range, one-use vehicles, such as munitions. Such devices may be employed to measure acceleration directly and rotation rate indirectly through the Coriolis principle. According to that principle, a body traveling at a velocity V subject to rotation .OMEGA. experiences an acceleration A.sub.c defined as the cross product A.sub.c =2.OMEGA..times.V. By imposing a sinusoidal relative velocity of the form: EQU V=V.sub.o sin .omega.t
The corresponding Coriolis acceleration then becomes: EQU A.sub.c =2.OMEGA..times.V.sub.o sin .omega.t
The measurement of rotation rate is obtained by determining the resultant Coriolis force exerted upon a deflectible force sensitive member.
Micromechanical devices are well suited for operation in low cost systems due to the compactness, simplicity and batch processing capabilities that they offer. In general, they feature a responsive element hinged to a frame along an edge (accelerometer) or a paddle that is rotatable about an axis defined by aligned flexure beams that support it with respect to a counter-oscillating hub (rotation rate sensor element). Pending U.S. patent application Ser. No. 09/127,375 of inventor Stanley F. Wyse entitled "Micromachined Rotation Sensor with Modular Sensor Elements" discloses inertial sensors of the foregoing type.
Systems, known as multisensors, that employ micromachined accelerometers of the type that include a hinged pendulous mass to measure both linear acceleration and angular rate (via the Coriolis principle) are taught, for example, in United States patents (property of the assignee herein) U.S. Pat. No. 4,996,877, entitled "Three Axis Inertial Measurement Unit With Counterbalanced Mechanical Oscillator"; U.S. Pat. No. 5,007,279, entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator"; U.S. Pat. No. 5,065,627 entitled "Three Axis Inertial Measurement Unit With Counterbalanced, Low Inertia Mechanical Oscillator;" and pending U.S. patent application Ser. No. 08/904,923 of Stanley F. Wyse entitled "Counterbalanced Triaxial Multisensor with Resonant Accelerometers". U.S. Pat. No. 5,614,742 of Thomas Gessner, et al. entitled "Micromechanical Accelerometer With Plate-Like Semiconductor Wafers" presents an example of an all-silicon, precision micro-mechanical accelerometer that comprises an assembly of five anisotropiocally-etched silicon wafers (each formed by a conventional wet process) bonded to one another to form a hermetically-sealed assembly. By forming a structure entirely of silicon layers coated with thin oxide layers, thermal coefficient mismatches are substantially overcome. As a result, the device is capable of withstanding a wide range of temperature variations with repeatability and stability. Pending U.S. patent application Ser. No. 09/127,643 of Robert E. Stewart, et al., property of the assignee herein entitled "Micromechanical Semiconductor Accelerometer", teaches an improvement on the patented device that employs SOI ("silicon-on-insulator") technology reduce parts count and complexity of assembly and manufacture of such a device.
While micromachined silicon devices of both the hinged accelerometer and paddle-like rotation sensor type provide essential elements of various inertial sensor systems, their very compactness subjects them to numerous sources of degradation. These include the presence of stray capacitance, unsatisfactory environmental shielding and designs that require bottom wire bonding access.
Analysis has shown that noise in the output of systems incorporating the above-described types of devices is a positive function of the ratio of stray to motional pickoff capacitance. A large value of stray capacitance can cause the pickoff amplifer to oscillate. Another source of error common to micromachined sensor devices results from incomplete shielding from the environment. This can result, in devices formed of multiple wafer layers, from the fact that a structure internal to a wafer layer, such as a paddle or pendulous mass, must be held at a non-ground potential. In such device, this prevents the grounding of exposed edge portions making them subject to undesired fluctuations of capacitance between the device and its housing or other apparatus upon repositioning, encountering surface deformations and the like. Such fluctuation negatively affects device performance and accuracy.
Among the problems inherent in the very small structures and working areas provided by micromachined devices are those arising from the need to maintain a hermetic seal of the cavity wherein the sensitive or responsive element resides while, at the same time, providing the shielding to avoid fluctuations and thereby enhance stability.
It is also highly desirable to provide devices that do not require bottom access to ohmic contacts, as this prevents the use of automatic bonding equipment as well as posing other difficulties. Such devices must typically be joined to a substrate. Conventional hybrid assemblies are not designed for bottom accessing.